Project Abstract Triple-negative breast cancer (TNBC) is a heterogeneous subtype of breast cancer with limited treatment options and frequently exhibits resistance to chemotherapy leading to poorer prognosis in TNBC patients. African American (AA) women in the U.S. suffer from higher incidence rates of TNBCs and poorer clinical outcomes as compared to their European American (EA) counterparts. These ethnicity-associated differences in outcomes remain significant after controlling for socioeconomic and treatment variations, suggesting that intrinsic differences in tumor biology exist between AA vs. EA TNBCs and contribute to outcome disparities. To decode the biologic factors contributing to the unequal TNBC burdens, we employed a novel systems biology approach (InFlo) to interrogate transcriptome profiles of TNBCs in AA and EA women, and identified MYC-MIZ1 signaling to be disparately activated in up to 70% of AA TNBCs as compared to only 30% of EA TNBCs. Patients harboring MYC-MIZ1 activated TNBCs exhibited poorer overall survival. Furthermore, we found higher levels of M2 Macrophage infiltration (>2-fold) in TNBCs in AA as compared to EA, suggesting that a tumorigenic immune microenvironment contributes to the disparate cancer burdens in AAs. These findings suggest for the first time, that ethnicity-associated differences in MYC-MIZ1 signaling and inflammatory immune modulation jointly contribute to TNBC outcome disparities. This proposal is fashioned to delineate the biologic role of MYC-MIZ1 signaling and M2 Macrophages in TNBC disparities as follows: Aim 1: Use multi-omics assessments performed in AA and EA primary TNBC tissues and cell line models to decipher the upstream regulators and the downstream mediators of MYC-MIZ1 signaling in TNBCs. We will decode the cross-talk between tumor MYC-MIZ1 signaling and M2 Macrophage infiltration in TNBCs using co- culture experiments. These studies will be the first to establish the regulatory roadmap of MYC-MIZ1 signaling in TNBCs and the mechanisms underlying functional crosstalk between the immune microenvironment and tumor MYC-MIZ1 signaling in AA versus EA TNBCs. Aim 2: Determine whether MYC-MIZ1 signaling modulates TNBC response to chemotherapy using genetic manipulation studies in preclinical cell line, xenograft and PDX model systems. These studies will unravel the functional role of MYC-MIZ1 signaling in conferring chemotherapy resistance in TNBCs, and revealing potentially novel therapeutic strategies in this disease. Aim 3: Evaluate the generality of frequent MYC-MIZ1 signaling activation and increased M2 Macrophage infiltration in AA TNBCs by using large-scale and diverse clinical cohorts of TNBCs derived from distinct AA and EA populations within the U.S. Additionally, we will test for the association of MYC-MIZ1 signaling and immune infiltration modulation with patient prognosis, clinicopathologic and/or lifestyle factors. Our studies will identify mechanisms contributing to chemotherapy resistance and racial disparities in TNBC outcomes, enabling development of prognostic biomarkers and targeted therapeutic strategies.